The present invention relates to a rubber-made covering member for push button switches or, more particularly, relates to a rubber-made covering member for push button switches having improved structure suitable for use in key-board switching units in various kinds of information-processing instruments such as computers, electric typewriters and the like operated by an operator over a length of time.
The switch operation is most of the prior art push button switches utilizes a metal-made spring member or a rubber-made covering member in the form of a truncated cone or a dome exhibiting more or less a snapping action or a clicking action. Therefore, the stroke vs. load relationships expressed by graphical curves in these push button switches are usually not uniform but, when the pushing stroke of the push button is gradually increased, the load on the push button at first increases linearly while a miximum is reached at a stroke value beyond which the load suddenly decreases to a minimum point and then increases again as the stroke is further increased over the minimum point of the pushing load. That is, the sudden decrease in the pushing load from the above mentioned maximum to the minimum is caused by the snapping action or clicking action of the resilient member in the push button switch. The working point of the switch is in general at or near the minimum point, i.e. at the end of the snapping action. The increase in the stroke thereafter is called an overstroke caused by the compression of a certain member in the switch.
It is a general trend that those push button switches having a rubber-made covering member are preferred to those having metallic resilient members when the switch operation involves a snapping action of the resilient member because the overall key stroke can be made larger in the former than in the latter so that a larger versatility is obtained readily in controlling the touch of key pushing in the most comfortable and reliable way (see, for example, Japanese Patent Disclosure 50-47179). The snapping action of the resilient member in these push button switches is very important in preventing or reducing the phenomena of chattering or bouncing in the switch operation and, in the same time, the operator of the push button switch can obtain a reliable touch of switching-on of the push button switch when the snapping action of the resilient member takes place. In this respect, the length of the key stroke before the snapping action takes place or, in other words, up to the maximum point in the key stroke vs. pushing load curve is an important parameter on which the mental as well as muscular fatigue of the switch operator largely depends to smoothly conduct the switch operation with a minimum number of errors over a long period of continued operation works.
The optimum length of the key stroke before beginning of the snapping action may depend on several parameters of the push button switches. In a conventional key board type push button switch provided with a rubber-made covering member, of which the pitch in the key arrangement is 14 to 20 mm, the full key stroke is 2 to 4 mm and the maximum pushing load necessary for switch operation is 40 to 200 g, for example, it has been empirically established that the optimum key stroke before the snapping action is 1.0 to 1.8 mm from the standpoint of decreasing the fatigue of the operator.
In the actual design of the conventional push button switches having a rubber-made covering member with pushing portions in the form of a truncated cone or a dome, the key stroke before the snapping action cannot be so large as desired but is usually in the range from 0.3 to 1.0 mm as restricted by the general structure thereof so that the operator of the key board switches receives an accelerated feeling of fatigue. With an object to mitigate the feeling of fatigue of the operator, an improved push button switch of the above described type has been proposed in which a coiled spring member is provided as an auxiliary resilient member used in combination with the rubber-made covering member. Push button switches of this type are, however, disadvantageous practically because of the increased number of the parts to be assembled to the switch as well as the increased laboriousness in assembling the parts into the switch including the coiled spring to be handled with burdensomeness resulting in inevitably increased costs for manufacturing.